Aluminum alloy



Patented May 6, 1941 Aluminum Company of America, Pittsburgh, Pa., acorporation of Pennsylvania No Drawing. Application September 28, 1940,Serial No. 358,925

3 Claims.

This invention relates to aluminum base alloys, and it is particularlyconcerned with wrought alloys that receive a solution heat'treatment andare artificially aged. This application is a continuation-in-part of mycopending applicationSerial No. 309,392, filed December 15, 1939.

It has been known. that aluminum base alloys containing more than percent of zinc, especially those that also contain copper, magnesiumand/or silicon, possess relatively highstrength in the wrought form.These alloys, however, sufier from several disadvantages, as compared toalloys containing copper and magnesium as the chief added alloyingelements but no zinc,

amount of zinc is used, increased difficulty in of. time. Another objectis to provide an aluminum base alloy of this type which can be readilyhot worked. Still another object is to provide a wrought solution heattreated. and artificially aged aluminum base alloy containing zinc whichexhibits substantially no acceleration of corrosion under stress.

a My invention is predicated upon the discoverythat aluminum base alloyscomposed oi. aluhot working, a lower resistance to corrosion, and

under certain conditions a susceptibility to 'a form of corrosion knownas stress cracking.

The early alloy compositions containing 15 per,

cent or more of zinc were later modified by the addition of suchelements as copper, magnesium, and silicon, and some reduction in theamount of zinc. While the reduction in the amount of zinc and theaddition of other alloying elements reduced the above nameddisadvantages, they were not entirely eliminated.

Some of the most satisfactory high strength aluminum base alloyscontaining less than 15 percent zinc together with additions ofmagnesium, copper, and manganese,. arethose disclosed in U. S. Patent1,924,729 to L. J. Weber.

structures which are designed to utilize the maximum loadingcharacteristics ofthe structural material, an intensive search has beenmade in the field of high strength aluminum basealloys to findcompositions that are relativehr free from susceptibility toacceleration of corrosion under stress.

The principal object of my invention is to provide an improved wroughtaluminum base alloy containing zinc as the principal added alloyingcomponent, which possesses both high strength and a high degree ofresistance to corrosion under stress. A further object is to provide anage hardened alloy of this type having stable mechanical properties overa long period minum, 4 to 6 per cent zinc, 0.75 to 2.5 per centmagnesium, 0.1 to 2 per cent copper, 0.1 to l'per cent manganese, and asmall amount of one or more of the grain refining elements titanium,boron, zirconium, molybdenum, tungsten, cobalt, chromium, and vanadium,avoid to a large extent the above named disadvantages usually associatedwith aluminum base alloys containing substantial amounts of zinc. By theexpression grain refining elements, I mean that these elements refinethe grain structure of the alloys in either the cast or wroughtcondition or both. At least one of the group of grain refiningelementsshould be employed in the following amounts: 0.02 to 0.25 per centtitanium, 0.005 to 0.1 per cent boron, 0.01 to 0.15 per cent zirconium,0.02/to 0.25 per cent molybdenum, 0.02 to 0.2 per cent tungsten, 0.02 to0.2 per cent cobalt, 0.05 to 0.5 per cent chromium, and 0.02. to 0.2 percent vanadium. Alloys falling within the foregoing range possessexceptionally high stable mechanical properties and resistance tocorrosion when thermally treated in the-conventional manner to improvetheir strength and hardness. I have also discovered that when thesealloys are artificially agedafter solution heat treatment they areremarkably resistant to the acceleration of corrosion under theinfluence of stress. Although many lower strength aluminum base a1- loyspossess a satisfactory resistance to ordinary corrosion, it has beenobserved that it is difiicult to obtain a high degree of resistance toacceleration of corrosion under stress in alloys having 1 very highstrength. By the expression acceleration of corrosion under stress}? Imean that upon exposure to the same corroding medium there issubstantially no increase in the susceptibility to loss of strength inan alloy article under external stress as compared to the susceptibilityto lossin strength in the same article under no external str'ess.

in a duralumin type of alloy. This property is of value in commercialheat treating operations,

- since variations in heating conditions can easily occur, and it istherefore desirable to use an alloy which is not too sensitive to suchvariations. In

the case of my improved alloy, the solution heat treatment may be givenwithin. a range of .820 to 1000 F. The period of time required to securethe desired solution of soluble constituents will, of course, varysomewhat with the temperature and with the mass of material beingheated, butthis is a matter that can be easily determined under theparticular conditions existing in commercial operation.

Another advantage possessed by my alloy is I that it may be quenchedfrom the solution heat treating temperature in a variety of quenchingmedia without/great eifect upon the ultimate mechanical properties ofthe alloy. In other words, my-"alloy is not as sensitive to variationsin a quenching procedure, especially if the copper-content does notexceed about 1 per cent, as aremany of the aluminum base alloys. In anumber of casesv an air blast quench can be used 1 whereas this cannotbe employed in quenching processes, such as rolling, forging orextruding.

Several examples may be cited to illustrate the properties obtainable inmy improved alloys. The A alloys tested had the following chemicalcomduralumin type of alloys ifmaximum strength and resistance tocorrosion is desired.

As mentioned hereinabove, my improved alloy must be artificially aged inorder to attain high stable mechanical properties and maximum re-isistance to corrosion under stress. Although my I alloy willspontaneously age at room temperature after beingquenched from'thesolution heat treating temperatureand exhibit a continuous increase instrength over a long period of time, I have found that the resistance toacceleration of corrosion under stress decreases with prolonged aging atroom temperature which is, of course, highly undesirable. If, however,the alloy is artificially aged instead of aging at room temperature, thedesired resistance to corrosion is obtained. The artificial agingtreatment should consist of reheating the quenched alloy to atemperature between 225 and 340 F. and holding it at that temperaturefor a period of 4 to hours. In general, I have found that aging at atemperature of 275 F. for 8 to 12 hours produces a very satisfactorycombination of properties.

In the manufacture of articles from sheet, it is often necessary to coldform the sheet. In the case of the solution heat treated and aged,duralu min type alloys, the age hardening occurs at room temperatureand progresses so rapidly after the alloy has been quenched that it isdifficult to cold form the alloy. In contrast to such a condition, myalloys age harden but slowly after having been quenched from thesolution heat treating temperature with the result that they may bereadily cold formed over a muchlonger period after quenching. After thecold forming operation the alloys are artificially aged in the mannerdescribed above. I have also found that even the hours at 320 F.Following these treatments,

artificially aged material can be readily cold formed as compared toother age hardened aluminum base alloys of theduralumin'type.

' In the manufacture of my alloys, aluminum is used'which contains theusual impurities of iron and silicon. In general, I prefer to use metalcontaining less than about 0.4 percent total of iron and silicon.However, it is possible to allow as much as 0.75 per cent silicon and0.5 per cent iron. Where the alloys are-cast by a continuous process,the silicon should not exceed 0.2 per cent.

Through use of the foregoing thermal treatments, I have been able toconsistently obtain in positions:

TABLE I Chemical composition, per cent Magne- Cop- Manga- 8111- v Tita-Anoy Zmc sium per nese con Iron nium All of the alloys were rolled tosheet 0.064 inch in thickness in the usual manner and thermally treatedas follows: -Sheets from alloys C, D, E, F,

and G were heated in anair furnace at 920 F.

for 20 minutes, quenched in cold water and aged at 275 F. for 12 hours.were heated at 970 F. for 20' minutes, quenched in cold water and aged12 hoursat 320 F.', while sheet from alloy B was heated at 970 F. for 20minutes, quenched in cold water and aged for 18 tensile'test specimenswere taken from the sheets of the several alloys for the several tests.The

average mechanical properties of these alloys were as follows: i

' TABLE II Mechanical properties Tensile Yield Elonga- Anoy strengthstrength tion Lbs/sq. in. Lba/rq. in. Percent Additional tensile testspecimens were subjected to an alternate immersion test for 48 hoursconsisting of elevating from and lowering I the 'specimens'into anaqueous solution of 5 per cent sodium chloride and 0.3 per cent hydrogenperoxide. One group of specimens from each alloy were exposed to thetest in an unstressed condition, while the other group were stressed assimple beams in an amount equal to per cent of'the yield strength. Atthe conclusion of the 48 hour period, the specimens were'removed andtheir mechanical properties determined. A comparison was made betweenthe properties of the corroded specimensand the original properties ofthe several alloys. The change brought about by corrosion is expressedSheets from alloy A,

in the table below in terms of the per cent lost with respect to theoriginal tensile strength and elongation values.

TABLE III Tensile test specimens from the material described above weresubjected to the 48 hour alternate immersion test referred tohereinabove, one portion of the test specimen being exposed 5 in theunstressed condition and the other porlosses tion being stressed 75 percent of the yield v strength. The per cent, change in propertiesUnstrcssed Stressed caused by corrosion are given in the table below. ATABLE V lggg ggg Corrosion losses -25 -7 -29 Unstressed Stressed l6 -3-18 1 -50 -6 -54 Alloy Quench .25 Tensile Elonga- Tensile Elonga- -7 1 7strength tion strength tion 38 -4 38 75 -10 -sa Air blast -14 -67 -24 88--.do -5 -7a -4 -70 It will be observed that there is littlediflerqggfigfiggg :2 :2; :2 :22 ence in loss of tensile strength betweenthe Oil n stressed and unstressed specimens. This indie cates that theapplied stress had substantially no It will be observed that alloy Cwhich conefl'ect upon the resistance to corrosion, and hence it may besaid that there has been no acceleration of corrosion by stress.

The efiect of the copper content of the alloy upon the resistance tocorrosion and acceleration of corrosion under stress when a less drasticquenching medium than cold water is used, is shown in the followingtests. For this purpose, sheets from alloys C, D, E, and G were used.The sheets from alloys C, D, and E were heated in an air furnace at 920F. for minutes and quenched in a high velocity air blast, the air beingat room temperature, and aged for 12 hours at 275 F. The G alloymaterial was likewise heated in an air furnace at 920 F. for 20 minutes,one portion being quenched in boiling water, and another portionquenched in a commercial quenching oil at room temperature.

Material from both portions were aged for 12 hours at 275 F. Themechanical properties of these alloys in this condition were as follows:

TABLE IV Mechanical properties tained more than 1 per cent coppersuffered proportionately greater losses than the other alloys containingless than 1 per cent copper. It is also evident that stressing thelatter alloys containing less than 1 per cent copper had substantiallyno eifect upon the resistance to corrosion.

Having thus described my invention, I claim:

1. A wrought, heat treated, and artificially aged aluminum base alloycomposed of from 4 to 6 per cent zinc, 0.75 to 2.5 per cent magnesium,0.1 to 2 per cent copper, 0.1 to 1 per cent manganese, and at least oneof the group of grain refining elements consisting of 0.02 to 0.25 percent titanium, 0.005 to 0.1'-per cent boron, 0.01 to 0.15 per centzirconium, 0.02 to 0.25 per cent molybdenum,- 0.02 to 0.2 per centtungsten, 0.02 to 0.2 per cent cobalt, 0.05 to 0.5 per cent chromium,and 0.02 to 0.2 per cent vanadium, the balance being aluminum.

2. A wrought, heat treated, and. artificially aged aluminum base alloycomposed of from 4 to 6 per cent zinc, 0.75 to 2.5 per cent magnesium,0.1 to 2 per cent copper, 0.1 to 1 per cent manganese, 0.05 to 0.5 percent chromium, and 0.02 to 0.25 per cent titanium, the balance beingaluminum.

3. a wrought, heat treated, and artificially aged aluminum base alloycomposed of from 4 to 6 per cent zinc, 0.75 to 2.5 per cent magnesium,0.1 to 2 per cent copper, 0.1 to 1 per cent manganese, and 0.02 to 0.25per cent titanium, the balance being aluminum.

- JOSEPH A. NOCK, JR.

